This invention relates generally to a device and method for mapping a three-dimensional surface and in particular to a device and method for generating and displaying a three-dimensional map of the anterior and posterior surfaces of transparent objects such as a patient's cornea.
Certain ophthalmic surgery procedures require the surgeon to know the shape and thickness of the patient's cornea. For example, when implanting vision correcting devices in a patient's cornea, insertion of the implantable device at the correct location and depth is critical.
The prior art discloses devices for obtaining information about the surfaces of a patient's cornea. Keratometers provide information about the anterior surface of the patient's cornea by projecting rings of light onto the cornea, then measuring the deflection of the light rings by the reflecting corneal surface. Examples of computer-based keratometers may be found in U.S. Pat. Nos. 4,685,140 and U.S. Pat. No. 5,110,200. One drawback of these devices, however, is that they provide information about the anterior corneal surface only. A second drawback is that these devices rely on specular reflectance of the cornea along an axis perpendicular to the cornea. Placement of the system optics directly in front of the patient's eye could interfere with the performance of surgical procedures on the eye.
Another prior art anterior corneal surface measuring device is shown in Lange U.S. Pat. No. 5,116,115. Lange discloses a system in which a flexible polymer sheet is draped over the anterior corneal surface. Lange projects patterns of lines onto the sheet and uses a camera to receive reflections of the patterns. Measurements of the shifting of the reflected lines is used to generate a map of the anterior corneal surface.
Another type of prior art device is exemplified by the NIDEK EAS-1000 anterior eye segment analysis system. This system uses Scheimpflug photographic techniques to obtain slit images of the patient's eye, i.e., cross-sectional images showing the anterior and posterior corneal surfaces along a single plane through the cornea. While the components of this system may be located away from the axis perpendicular to the eye and will therefore not interfere with any eye surgery, the NIDEK system cannot be used to generate maps of the entire anterior and posterior surfaces of the patient's cornea in a clinically useful time.
Gormley et al. describe a corneal modeling system in "Cornea Modeling," 7 Cornea 30 (1988). The Gormley system uses two separate optical subsystems to obtain information regarding the topography of the anterior corneal surface and the thickness of the cornea, i.e., the distance between the anterior and posterior corneal surfaces. The first optical subsystem is a cylindrical photokeratoscope that projects a 32-ring image onto the anterior surface of the cornea. Reflections of the projected ring pattern are used to generate a topographic image of the anterior corneal surface. The second optical subsystem is a dual-beam scanning laser slit lamp that obtains 14 optical cross-sections of the cornea. The two sets of data points are used to generate a map of the anterior and posterior surfaces of the cornea.